The management of heat and energy on current military aircraft has become increasingly challenging due to the array of sensors, electronics, and weapons systems onboard. The ability of an aircraft to accomplish a thermally stressful mission can be limited by the requirement to remove heat from aircraft subsystems. Logistically complex pre-flight measures may need to be employed to enable aircraft to complete thermally stressful missions. Thus, high efficiency subsystems, as well as power and thermal management systems, are needed to mitigate such problems.
Flight control actuation systems are now contributing more to the heat load on aircraft since systems have moved from centralized hydraulic systems, to decentralized electro-hydrostatic actuator (EHA) systems. Future aircraft are expected to make additional use of electromechanical actuators (EMAs), which will further increase thermal loads on the aircraft energy management system. The use of composite skins on new aircraft has made it more difficult to dissipate waste thermal energy to the atmosphere. The problem is particularly relevant to passively cooled electrical components, such as electromechanical flight control actuators, that are isolated inside of internal bays inside of composite wings and the aircraft empennage.
A typical EMA may include an electric motor, gearbox, screw, nut, and linkage. EMAs employing a nut with a recycling raceway of ball bearings may be referred to collectively as a ball screw. In these configurations, either the nut or the screw can serve as a ram, depending upon which element is held fixed relative to the aircraft fuselage. The ram may be connected to a linkage that forces the aerodynamic surface to pivot. The motor current required to produce the surface motion commanded by the flight control system is proportional to the torque that is required to overcome the inertia and aerodynamic loads applied to the system. The rate at which waste heat is generated resulting from inefficiencies inherent in the actuator is proportional to the square of the applied current and the effective friction coefficient of the moving components. Since motor torque is proportional to current, minimizing waste heat is equivalent to minimizing motor torque.
Mechanisms may be employed to reduce or eliminate the torque necessary to maintain the EMA's position while under load. For example, a self-locking screw may be used in an EMA. In such self-locking configurations, the drive motor is not required to generate any torque to hold a load in a fixed position. As a result, holding the position does not require current or power, thereby eliminating waste heat. However, when a self-locking screw and nut are in motion, or when overcoming potentially significant stiction forces to initiate movement, the high friction coefficient results in elevated levels of heating as a result of the motor current required to overcome friction. Waste heat resulting from those additional frictional losses is transferred to the fuselage. Conversely, if a screw is not self-locking, motor current is required to hold a fixed position when an external load is applied. Most EMAs designed for use on aerospace vehicles use non-self-locking ball screws in order to satisfy safety of flight requirements (dictating that surfaces must fail in a floating rather than a locked position). These non-self-locking ball screws also require much less power to operate when they are in motion when compared to self-locking screws, as their equivalent friction coefficients are an order of magnitude lower than the friction coefficient of the best self-locking power screws. Unfortunately, since they are so efficient at turning rotational energy into linear motion, linear loads are conversely transformed into a rotational load that must be resisted by the motor.
As a result of the above noted deficiencies, there is a need in the art for a selectively self-locking EMA that requires reduced or zero motor current to hold static loads, while taking advantage of the low friction benefits of a ball screw when the actuator is in motion.